The use of SF.sub.6 as an echant gas is shown in U.S. Pat. No. 4,330,384. U.S. Pat. No. 4,447,290 also shows the use of SF.sub.6 with the addition of Freon 12 as a indicator gas for the plasma etching of polysilicon. Neither of these patents, however, is concerned with the problem solved by the present invention.